1. Technical Field
The present invention is directed to a thruster for propulsion of a mass, and in particular to a pulsed plasma thruster for propulsion of a mass.
2. Background Art
Pulsed plasma thrusters used to propel spacecraft and satellites in outer space are old in the art. In a plasma thruster, a stream of partially or fully ionized gas particles, called a plasma, is used to produce thrust. In a pulsed plasma thruster, an electric arc generated by a pulsed electric discharge applied between two electrodes is used to heat a propellant material to produce the ionized gas particles or plasma.
The propellant material is conventionally either in a gaseous state or a solid state prior to heating. In a gas-type plasma thruster, an electric arc is conventionally passed through the gas to ionize the gas and produce plasma. See U.S. Pat. Nos. 5,425,231; 4,821,508 and 3,425,223. In a solid-type plasma thruster, the electric arc is passed over the surface of the solid material to increase the temperature of the material, thereby ablating the material to form an ionized gas. See U.S. Pat. No. 3,447,322; G. Paccani, Anode-Nozzle Experimental Analysis in a Coaxial Non-Steady Solid Propellant Thruster, Deutsche Gesellschaft fuer Luft-und-Raumfahrt (1988); and D. Palumbo and W. Guman, Effects of Propellant and Electrode Geometry on Pulsed Ablative Plasma Thruster Performance, AIAA 11th Electric Propulsion Conference, AIAA Paper 75-409 (1975).
It is desirable to exhaust the plasma from the thruster to produce thrust and to perform useful work, such as to change the orientation or speed of a spacecraft mass to which the thruster is attached. The plasma may be exhausted from a chamber under the pressure caused by the heating and/or ablation process (electrothermal effect). Alternatively, the flow of electric current between the first and second electrodes may cause the ionized particles to be exhausted under the influence of the electric and magnetic fields caused therebetween (electromagnetic effect).
The production of plasma in conventional pulsed plasma thrusters, however, has come at a significant cost. Typically, conventional pulsed plasma thrusters have had an efficiency of less than 10%. See R. Vondra et al., Analysis of Solid Teflon Pulsed Plasma Thruster, Journal of Spacecraft (1970). That is, less than 10% of the total energy consumed by the thruster is converted to a form (thrust) which can be used to perform work. The low efficiency of these plasma thrusters has been a significant disadvantage, considering that energy consumption has always been a critical consideration in spacecraft design.
Palumbo and Guman describe a number of pulsed plasma thrusters which are claimed to have improved efficiency. In these thrusters, an electric arc is generated between two oppositely charged plate electrodes to heat two solid propellant bars disposed therebetween. With heating, the surfaces of the solid propellant rods are ablated, and an ionized gas is formed in a cavity or space defined by the surfaces of the electrodes and the bars. In all of the thrusters shown, the current path of the electric arc between the oppositely charged plates is perpendicular to the flow path of the gas generated by ablation of the solid propellant. Furthermore, in all of the thrusters shown, the space or cavity between the propellant bar surfaces is on the order of 1.7 cubic inches (28 cubic centimeters).
Paccani also describes a pulsed plasma thruster claimed to have improved efficiency. This thruster uses a coaxial arrangement of electrodes to heat and ablate a propellant material disposed therebetween to produce plasma. In all of the thrusters shown, the current path of the electric arc between the oppositely charged electrodes is substantially perpendicular to the flow path of the plasma produced from the solid propellant. Furthermore, after the plasma has passed through the space between the two electrodes, the plasma is exhausted out of a nozzle which is made of conductive material and formed integrally with one of the coaxial electrodes.
U.S. Pat. No. 3,447,322 describes a pulsed plasma thruster having first and second electrodes disposed at alternate ends of a cavity formed of a propellant material. The propellant material is heated in the cavity by an electric arc generated between the electrodes to produce plasma. The plasma is exhausted out of a nozzle which is made of conductive material and formed integrally with one of the electrodes. It is believed that the electric discharge is pulsed such that the gas is exhausted through the nozzle during the periods when no electric discharge occurs between the electrodes.